P. William Filby's _Directory of American Libraries with Genealogy or Local
History Collections_ (Wilmington, Delaware: Scholarly Resources, 1988) is a
little dated, but lists a lot of the major libraries that returned a
questionnaire about their genealogical holdings.

Access to Ancestry.com is not a question of "allowing" but of the library
paying for a subscription to Ancestry.com. In preparation for a talk that I
will be giving in May on Genealogical Collection Development for libraries,
I'm working on getting in touch with the company that provides subscriptions
to AncestryPlus for libraries to get more information about what exactly
they provide, and pricing schedules. From what I've seen so far, it appears
that somewhere on the Internet, there's a site that lists the libraries, but
I haven't gotten as far as finding it. I'm hoping to have a chance to call
them tomorrow, and if I can locate this list, I'll let you know.